March 2016 Tech Challenge – Answer


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A rooftop utilizing R-410A with a fixed orifice metering device has a customer complaint of “no cooling.” The symptoms confirm that the thermostat is not satisfied and the condensing unit and blower are running continuously. The system has only had maintenance, and no service issues. What are the “possible causes” (there is only one problem intended) with the following measured conditions on this field service check sheet?

Field Service Check Sheet

Compressor Discharge Temp. 219 deg. F.
Condensing Press./Temp. 281 psig / 93 deg. F.
Condensing Outlet Temp. 87 deg. F.
Condenser Sub cooling 6 deg. F.
Condenser Split 8 deg. F.
Entering Feed Device Temp. 86 deg. F.
Evaporator Press./Temp. 145 psig / 51 deg. F.
Evaporator Outlet Temp. 65 deg. F.
Evaporator Superheat 14 deg. F.
Compressor Inlet Temp. 76 deg. F.
Total S. H. 25 Deg. F.
Ambient Temp. 85 deg. F.
Room/Box Temp. 80 deg. F
Compressor Volts 240 V.
Compressor Amp. Draw Low

And the answer is…

The problem appears to be a partial restriction ahead of the feed device. The temperature entering the feed device is lower (49 deg. F.) than the ambient temperature of 88 deg. F., caused by the partial restriction boiling of refrigerant (evaporation). The likely problem is a partially blocked filter/drier. The low side of the system is starved and is not picking up heat in the evaporator so the high side conditions are low also with low pressures and splits. The superheat is high causing a high discharge temperature and the condensing subcooling is with-in one degree of ambient and appears to be normal because of the low refrigerant flow.


Posted In: ACCA Now, Tech Challenge

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